The lubricating oils in question within the scope of the present invention consist essentially of naphthenic hydrocarbons and paraffinic hydrocarbons and their mixtures, the kinematic viscosity of these oils at 100.degree. C. being between about 1 and about 20 and, preferably between about 2 and about 10 mm.sup.2 /s.
There are already known copolymers of the kind in question which can be employed as multipurpose agents and especially as viscosity improvers in lubricating oils.
It is found, however, that, as oil manufacturers' requirements become increasingly constraining, copolymers which are already known no longer meet the requirements of practice, especially in their applications in so-called top-grade oils or in the case of highly specialized uses.
In addition, the multitude of factors that affect the properties of the above-mentioned copolymers and the large number of possible combinations of these factors mean that the search for copolymers capable of imparting ever better properties to lubricating oils, especially in respect of their viscosity, becomes increasingly difficult.